Some of us still use maps, it's not a bad idea to have one handy when visiting Mammoth.

Maps are available for purchase online, but one today and scout out the trails before arriving to town.

We recommend the Inyo National Forest (North), and the Inyo County map.

The best place to pick up some maps is the new "California Welcome Center" on the way into Mammoth Lakes on the right hand side.  The Welcome Center is staffed with USFS personel and Visitor Agents, none of whom ride or personally or know that much about the dirt routes in the area.  But don't let that stop you from going in and getting information as the center is loaded with maps, some free and most for purchase.
First we'll cover the free maps in the Welcome Center:
1. Off-Highway Vehicle and Mountain Biking Map (FREE) - this map is available as a large color tear-off and it is helpful by showing visitors about 5-10% of what's out there on a map.  Our two main OHV loops are noted and described, both the Look-out Loop and the Crater Loop.  This map is great for locating other area attractions that shouldn't be missed like hot springs, Devil's Postpile, Convict Lake, June Lake, Rock Creek, free campgrounds, fishing destinations, etc.  The mountain biking loops listed are for suckers, especially the Panorama Loop with it's ankle deep sand and pumice -- motorized even has a tough time slogging through that mess.
2. Motor Vehicle Use Maps, Inyo National Forest, also known as "MVUM" (FREE) - a series of seven or eight forest maps, cheaply printed on newsprint and for some reason you have to request it.  (they are kept top-shelf behind the counters)  These maps were designed in January 2010, but kept under wrap until May 2010.  If you are staying and want to see what legal routes are available in and near Mammoth, I'd suggest asking for the June Lake & Mammoth map, Bald Mountain & Little Antelope Valley map, and the Casa Diablo & Glass Mountain map.  Now these maps go into more detail on area routes and are supposed to be the end-all when it comes to what's out there -- unfortunately is falls short of that mark.  I'd say these maps show about 70% dirt routes and about 2% of singletrack.  Heck, they'll even show singletrack on the map that is nothing but a jeep trail.  I think the major reason between the discrepancy between these 2010 maps and what currently out there is the other (non-mapped) routes are all slated for closure, or, they were just plain missed at the last road inventory.
Regarding the MVUM's: because of their cheap design and overkill on meaningless tables, rules, descriptions, etc they are hardly usable for the average map reader and often you'll need a commercial map to accompany them in order to understand whats going on in the forest.  Which brings up the next topic.
Other recommended maps that cost a few bucks:
Inyo National Forest map - very good for planning and broad overview of the area.
USGS Topo maps - lots of detail but not good in the field due to limited size, we use software, AllTopo, and have the entire state of CA in 7.5 minute topo maps.
BLM Land Use Series - often the best map around, the Welcome Center doesn't carry the full line so stop off at the Jawbone BLM station where they do.
Other area maps like the SierraMap series, Harrison maps, or National Geographic topos - all are great. 
A note on dirt roads up here - and at last count we have about 4,000 miles worth:
Are all dirt roads green sticker friendly?  Our answer is yes, ask someone else they might say yes or no, or it depends.  Apparently the USFS has several dirt road classifications that they can't even keep track of or know 100% of what info given out is accurate.  If in doubt, whip out your MVUM.  Haha!
Lastly, Inyo and Mono are green and red sticker all year long!
Mapping update 12-14-2010

Club members have been participating in the design of a Mammoth and Bishop based CTUC (California Trail Users Coalition) map scheduled to be out early next year. It has the same look and feel as the Friends of Jawbone map, and covers a wide area. We wanted to include as much as possible to fully represent our area, but ultimately it isn't our map. (we are content providers)

Another map was just released for our area too and we recommend it. It's called the Eastern Sierra Recreation map and it covers riding areas from Lone Pine to Bridgeport and inset to the map are many smaller city and town maps. It's $6.95 and worth every dollar.